1. Field of the Invention
The present invention relates to a superconducting device which utilizes a superconductor adapted to fall into a superconducting state at a predetermined temperature, and more particularly to a superconducting device which can operate as a transistor.
2. Description of the Related Art
As a switching device which utilizes a superconductor, the Josephson device has heretofore been known. When compared with conventional semiconductor transistors, the Josephson device has the merits of a higher switching speed and a lower power consumption (several .mu.W). However, this Josephson device is basically a two-terminal device. It has consequently been impossible to exploit circuit techniques accumulated with the conventional semiconductor transistors, for the circuit arrangement of the Josephson device. Another drawback has been that the Josephson device must use an A.C. power source.
As a solution to these drawbacks, a superconducting device in which a superconductor and a semiconductor are combined is disclosed in the official gazette of Japanese Patent Application Laid-open No. 106186/1982. This superconducting device is constructed of a pair of superconductor layers which are disposed on a semiconductor substrate, and a control electrode which is disposed through an insulator layer in a part where the end faces of the superconductor layers oppose to each other. In this prior-art example, the superconducting device is fabricated in such a way that a superconductor layer formed on the semiconductor substrate is separated into two portions by photolithography and etching, to form the pair of superconductor electrodes, and that the separating part is thermally oxidized to form the insulator film, on which the control electrode is formed by evaporation.
In addition, a superconducting transistor (JOFET: Josephson Field Effect Transistor), the operating principle of which is such that the value of a superconducting current flowing across two superconductor electrodes disposed in contact with a semiconductor is controlled by changing a superconducting proximity effect with a voltage applied to a control electrode, is discussed in "Journal of Applied Physics," Vol. 51 (1980), p. 2786 by T. D. Clark.
The above technique is intended to realize a superconducting device of the field effect type. In fabricating the superconducting transistor of this type, a pair of superconductor electrodes confronted at a fixed distance l are disposed on a semiconductor substrate, and a control electrode is disposed in the confronting part. The value of the distance l is selected at about 5-10 times a coherence length .xi..sub.n in the semiconductor. When the value l is smaller than this range, the coupling between the superconductor electrodes is too intense, and the control by the control electrode becomes difficult, to incur by way of example the problem that the gain of the superconducting transistor lowers. In the superconducting transistor, accordingly, the opposing parts of the two superconductor electrodes cannot be brought closer than the certain value determined by the material of each semiconductor, from the viewpoint of the characteristics of the transistor. Meanwhile, the gate capacitance of a field effect transistor depends upon the length of a channel. In the superconducting transistor of the prior art, the distance between the two superconductor electrodes is substantially equal to the length of a channel. With the prior art, accordingly, the gate capacitance of the superconducting transistor cannot be rendered less than a fixed value, and this has led to the problem that the operating speed of the superconducting transistor closely related to the gate capacitance cannot be expected to rise.
Besides, the following literatures on superconducting devices have been known:
A) IEEE Trans. Magn., vol. MAG-15, pp. 435-438, 1979 PA1 B) IEEE Trans. Electron Devices, vol. ED-28, pp. 1394-1397, 1981 PA1 C) Official Gazette of Japanese Patent Application Laid-open No. 176781/1982 PA1 a semiconductor body; PA1 a pair of superconducting electrodes formed in contact with said semiconductor body; PA1 impurity regions formed within said semiconductor body, which are individually in contact with the corresponding ones of said pair of superconducting electrodes, and PA1 means to control current flowing across said pair of superconducting electrodes, PA1 the current controlling means being formed on said semiconductor body.
The aforementioned JOFET published by T. D. Clark includes electrodes made of a superconductor on a heavily-doped semiconductor substrate, and has its device characteristics controlled by a control electrode disposed on the semiconductor substrate through an insulator layer. Si.sub.3 N.sub.4, SiO.sub.2 or the like formed by CVD is employed as the insulator layer for electrically isolating the control electrode from the pair of superconducting electrodes.
Since the prior art forms a gate insulator layer and an inter-layer insulator film by the use of a method of evaporating SiO.sub.2, Si.sub.3 N.sub.4 or the like, the thickness of the gate insulator layer becomes unnecessarily great. Therefore, a voltage in excess of several volts must be applied for controlling the characteristics. This has led to the disadvantage of the prior art that a circuit gain exceeding unity cannot be achieved.
Moreover, in the fabrication of the superconducting electrodes, the deposited inter-layer insulator film and the superconducting electrodes need to be simultaneously fabricated. Therefore, the prior art has had the problem that the total thickness of the materials of the parts to be processed becomes very great and that a satisfactory fabrication precision is difficult to be attained.